Boron nitride (“BN”) comes in a variety of crystalline structures and has a variety of uses from polishing agents to lubricants. Hexagonal boron nitride (“hBN”) is a very desirable form of a white composition having hexagonal layer structure similar to graphite in platelet morphology. Because of its properties, it has found uses in heat conductivity applications, electrical insulation applications, corrosion resistance applications, lubrication applications, and as a plastic additive. Boron nitride can be molded and used in composite materials or as a raw material for cubic boron nitride. It is also used in many applications including electronic materials, non-oxidizing ceramics sintering filler powder, makeup materials, medical additives, etc.
In the prior art, BN may be manufactured in a high temperature reaction between inorganic raw materials forming a white powder composition of BN particles. When platelet BN is added as a filler to a polymer, a blended material is formed having poor Theological properties. At loaded concentrations above 30 wt. % BN, the blended material is so viscous that it is difficult to dispense from a mechanical dispenser such as a syringe. U.S. Pat. No. 6,731,088 discloses a process to manufacture BN, forming a dry powder of spherically shaped agglomerates of irregular non-spherical particles bound together by a binder and subsequently spray-dried. The spherically shaped BN agglomerates can be compounded into polymer compositions at levels of 35-50 wt. % for compositions with viscosity below about 300 cp.
JP Publication No. 05-051540 discloses BN powder treated with at least a titanate coupling agent, a silane coupling agent, and a non-inonic coupling agent in an amount of 0.1 to 5 wt. %, to improve the wettability of the BN in applications such as a release agent, a lubricant, a low-friction material, a coating material, etc. U.S. Pat. No. 6,162,849 discloses a thermally conductive moldable polymer blend having at least 60 wt. % of BN powder having an average particle size of at least 60 microns and coated with a coupling agent, and wherein the thermally conductive composition has a thermal conductivity of at least 15 W/m° K.
There is still a need for improved BN compositions, particularly for BN compositions that can be used in large quantities as a filler in applications including but not limited to automotive and electronic applications. Applicants have found a method for coating BN by first introducing reactive sites on the BN surface prior to the subsequent surface functionalizing step. The two-step coating process provides a more stable surface coating over the coating methods in the prior art, allowing the coating material to adhere to the surface of the BN as compared to some of the processes in the prior art.